The emission of sulfur compounds must be carefully controlled in light of federal regulations designed to curtail industrial pollution. Title 1 of the Clean Air Act Amendment of 1990, for example, continues the requirement that new facilities meet stringent criteria regarding the discharge of airborne sulfur compounds. In the case of a petroleum refinery sulfur recovery plant, which often comprises a Claus process coupled with an incinerator, the legally acceptable concentration of discharged gaseous sulfur dioxide is only 0.025% by volume. Besides Claus tail gas streams, another significant source of gaseous sulfur oxide emissions is sulfur containing fuel used in boilers, internal combustion engines, and heating units that produce contaminated stack and exhaust gas streams. Additionally, the smelting of sulfur-containing ores, coke refining, and sulfuric acid production all generate sulfur containing gaseous wastes.
Of significant commercial importance, then, are processes for the removal of sulfur compounds from such streams prior to their discharge. Typically, in the case of incinerator effluent streams, gaseous sulfur compounds are overwhelmingly in the form of sulfur oxides (e.g. sulfur dioxide and sulfur trioxide). Otherwise, hydrogen sulfide, carbonyl sulfide, carbon disulfide, and even elemental sulfur may also be present. To date, numerous processes have been developed to address the removal of these sulfur contaminants from waste gases. Perhaps most prevalent commercially is the use of a wet lime/limestone scrubbing agent to selectively extract sulfur compounds into an aqueous liquid as a mixture of sulfite and sulfate compounds of calcium.
Alternatively, a caustic scrubbing solution may be used to convert gaseous sulfur compounds into soluble sulfite and sulfate compounds. For example, if a sodium hydroxide scrubbing solution is used, the sulfur compounds will be sequestered in the liquid phase predominantly as sodium sulfite and sodium sulfate. In order to eventually dispose of such a liquid, however, it is important to limit the quantity of sulfite anion because sulfite will normally convert to sulfate and, in the process, deprive the effluent stream of biologically needed dissolved oxygen. Thus, it has been an ongoing objective in industry to not only remove sulfur contaminants from gas streams, but also to allow the disposal of liquid products resulting from this removal. These liquid products must therefore have sufficiently low chemical oxygen demand (COD) and biological oxygen demand (BOD).
To this end, several prior art references have focused on gaseous sulfur contaminant removal through a combination of adsorption and oxidation. For example, in U.S. Pat. No. 3,914,387, SO.sub.2 in a dust-containing waste gas (e.g. effluent from metallurgical operations) is removed via contact with a liquid NaOH/Na.sub.2 SO.sub.4 solution. The formation of sulfates, rather than sulfites, is highly favored due to the presence of Fe, Co, Ni, Mn, and/or V in the dust particles in quantities effective to catalyze the oxidation of sulfites in the presence of oxygen. However, although it is convenient to use gas-entrained metals for the necessary oxidation, uncertainties related to the quantities of such metals can make operation problematic. For example, metal addition to the gas stream or scrubber solution may be required if insufficient quantities for sulfite oxidation are inherently present. Alternatively, an excessive amount of metal-containing dust may present solubility problems and require means for filtration of the scrubber solution.
In a similar manner, U.S. Pat. No. 3,920,794 recognizes the utility of NaOH and Na.sub.2 CO.sub.3 scrubbing solutions for removing SO.sub.2 from gas streams. After the adsorption or scrubbing step, an oxidation is performed to convert sulfites to sulfates. This oxidation is carried out by the addition of catalytically effective metals (e.g. Fe, Cu, Co, Mn, and/or Ni) to the adsorption solution in the form of sulfate, phosphate, or chloride salts. Such salts are known to be soluble in sufficient quantities to catalyze the sulfite oxidation reaction. In this disclosure, it is noted that a secondary oxidation step may be required if the level of sulfites in the scrubbing solution after adsorption of SO.sub.2 is excessive, even with the addition of catalytic metals. Furthermore, the metal salts must be continually added to the scrubbing solution to replenish quantities lost through the disposal of spent liquid effluent.
In U.S. Pat. No. 4,156,712, the absorption and oxidation are combined in a single vessel through the simultaneous introduction of oxygen-containing gas into a lower portion of an aqueous liquid adsorption solution and the SO.sub.2 contaminated gas into an upper portion. If the adsorbent is Ca(OH).sub.2 or CaCO.sub.3, the sulfur impurities, in the oxidizing environment contemplated in this case, can be precipitated as sulfates. Unfortunately, a sufficient mass transfer rate of oxygen into the liquid phase is difficult without mechanical agitation. Also, the liquid adsorbent sulfate level is constrained to a small amount (less than 1%) to maintain a thermodynamically favorable environment for continual sulfate formation; otherwise, it is taught that an oxidation catalyst in the form of Fe.sup.+3 ions should be added. Other considerations associated with such a single-stage non-catalytic oxidative adsorption include the significant vaporization or even entrainment of adsorption solution caused by the large air flow requirement. Lastly, the temperature of the contaminated feed gas should be maintained considerably lower than a typical incinerator gas stream effluent, with 40-70.degree. C. being preferred.
In U.S. Pat. No. 4,307,069, flue gas containing SO.sub.2 is purified while the sulfur content is recovered in elemental form. The process is carried out through a sequence of steps, the first two of which are adsorption and oxidation. Adsorption is achieved with a scrubbing solution of NaAlO.sub.2 and NaOH to form a suspension, after contact with SO.sub.2, that is a mixture of sodium and aluminum sulfates and sulfites. To drive the oxidation of existing sulfites to sulfates, it is taught to add preferably ferric and ferrous sulfates to the suspension in the presence of oxygen. It is unlikely that these iron containing compounds can be easily recovered in a usable form, given that the suspension wherein they are dissolved is thereafter subjected to a reduction step in the presence of coal and air at 1000.degree. C.
Finally, in U.S. Pat. No. 4,452,766, another multi-stage process for the removal of SO.sub.2 from waste gas is described. Under circumstances where the gas stream is derived from a coal fired plant and contains fine particles of alkaline ash, effective adsorption can be attained without the continual addition of alkali or other scrubbing agent. Included in the overall method is an oxidation step wherein alkali bisulfite, the reaction product of adsorbed SO.sub.2 and alkali metal impurities in the gas feed stream, is converted to the sulfate form. This is achieved to some extent using the small amounts of oxygen contained in the flue gas that are dissolved in the scrubbing solution. However, the disclosure indicates that, for complete oxidation, it may be necessary to increase the amount of air in the boiler or furnace from which the waste gas is derived. In effect, a greater air flow to the adsorption medium to cause the oxidation invariably either increases solution losses through vaporization or results in higher energy requirements associated with liquid recovery.
In contrast to the aforementioned references, the present invention provides a method for the removal of SO.sub.2 and various other sulfur containing compounds in waste gas streams, whereby an oxidation step using a fixed bed of catalyst is performed. The oxidation provides an effluent liquid containing only small amounts of sulfites; thus, this effluent has only minimal biological and chemical oxygen demands and may, in a preferred case, be directly disposed of in an environmentally sound manner. Because the oxidation catalyst is maintained as a solid phase, losses of valuable catalytic metals are minimized. Furthermore, the oxidation reaction is effective enough so that essentially complete oxidation is achieved without the need for oxygen beyond that which is soluble in the liquid. An additional advantage of the present invention, aside from greatly reduced or eliminated solution and catalyst losses, is the ability of the novel scrubber vessel design to handle waste gas streams with elevated temperatures, characteristic of incinerator effluents. Lastly, the fabrication costs of equipment associated with this gas treatment method are relatively low, since both the required adsorption vessel and oxidation reactor are simple designs. Overall, therefore, the method of the present invention overcomes many of the limitations that the prior art has failed to address.